Optical information recording media such as magneto-optical recording media and phase-change recording media have been known as recording media that allow mass-storage and high-speed information recording, and further, information rewriting. Such an optical information recording medium utilizes, for information recording, changes caused in optical characteristics of its recording material by partial irradiation with a laser beam. For instance, a magneto-optical recording medium utilizes, for information recording, different rotational angles of polarization planes in reflected light caused by different magnetization states. A phase-change recording medium utilizes, for information recording, a phenomenon in which a reflected light quantity with respect to light with a specific wavelength differs between when the medium is in a crystalline state and when it is in an amorphous state. The phase-change recording medium is capable of erasing information and overwriting information simultaneously by modulating an output power of a laser beam, thereby allowing information signals to be rewritten at a high speed readily.
Such optical recording media have significant merits such as random accessibility in response to needs and excellent portability, and hence has more significance in the highly information-oriented society. They are used, or considered for use, for various purposes and in various fields: for instance, for recording and storage of personal data and video information by means of computers, in medical fields, in academic researches, as a portable recording medium for use in a digital video recorder, for replacement of video tape recorders for home use. A typical example of an article in which a phase-change recording material is used is a DVD-RAM, which is randomly accessible. This is a disk-form medium with a diameter of 120 mm that has a storage capacity of 2.6 GB on one surface (5.2 GB in the case of a lamination type). Now further expansion of the storage capacity (high densification) and further increase in speed are demanded as to these optical information recording media, in response to improvement in performances of applications and image information.
Examples of means to achieve high densification that have been proposed conventionally include laser with a shorter wavelength and a higher numerical aperture for a laser beam. Each of these makes it possible to decrease a minimum size of a laser beam spot, thereby enabling high densification of recording in a direction parallel with the laser scanning direction.
As another technique for achieving high densification, a so-called multi-layer recording medium technique has been proposed, in which a medium having two or more information layers that are provided on top of the other with a transparent separation layer interposed therebetween is used and all the information layers are accessible with a laser from only one side. This technique allows the storage capacity to increase in the medium thickness direction.
Conventionally, a typical emission wavelength of a laser beam has been obtained at a red range (for instance, a specific value in a range of 650 nm to 860 nm), and a laser at a wavelength in this range was low-cost and available readily. Therefore, to realize an optical information recording medium for use with this a laser, a recording material has been developed that exhibits adequate light absorption with respect to the red wavelength range and exhibits a significant change in the optical characteristics.
In these days, however, lasers at wavelengths in a blue-violet range (for instance, wavelengths of 300 nm to 450 nm, hereinafter referred to as “blue wavelength range”) that enables higher density recording have been developed, even to a merchandise level in terms of technologies. A technique of obtaining light with a wavelength half the wavelength of a laser beam by means of a second harmonic generation (SHG) element has been developed as well. This technique allows a laser beam with a wavelength of 410 nm to be obtained by using a laser with an oscillation wavelength of 820 nm. In this case, a recording material having an excellent optical characteristic in the blue wavelength range is required, but a recording material optimized for the conventional red wavelength range does not necessarily exhibit excellent characteristics in the blue wavelength range.
When a recording material with light absorption characteristics optimized for the red wavelength range is used to form a light-transmitting-type information layer on a side closer to the laser-incident side, in particular, the layer exhibits increased absorption of laser in the blue wavelength range, thereby making it difficult to improve a transmittance of the information layer. On the other hand, an attempt to improve the transmittance of the information layer makes it difficult to achieve a sufficiently great difference between optical characteristics of the information layer.